The present disclosure relates generally to automobile steering systems and, more particularly, to a method for implementing a vehicle stability enhancement (VSE) system reference model for compatibility with variable ratio (VR) steer control used in active front steering (AFS).
Vehicle Stability Enhancement (VSE) systems currently in existence typically employ a “Reference Model” algorithm that calculates a desirable handling motion for the vehicle. Such reference model algorithms have traditionally assumed that the desirable handling motion of the vehicle is solely a function of handwheel angle and vehicle speed. However, with the advent of newer vehicle systems such as active front steering (AFS) and steer-by-wire (SBW), the desirable handling motion is no longer simply a function of handwheel angle and vehicle speed.
In an AFS system, there is the ability to dynamically change the overall steer system ratio between the handwheel and the steered road wheels. The term “active steering” relates to a vehicular control system in which a generated output is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. In some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle.
For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels. In other words, a motion of the handwheel may be supplemented by an additional motion, such as that from a differential steering actuator, which in turn translates into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the handwheel. Accordingly, in order to provide closed-loop stability control for an AFS system, the VSE reference model should take into account any changes as a result of the VR control.